1. Field of the Invention
The present invention relates to hearing aids. The invention further relates to a system for measuring the occlusion effect by a hearing aid. The invention, still further, relates to a method for measuring the occlusion effect by a hearing aid in situ.
Occlusion Effect
When a hearing aid is placed in the ear of the user with an acoustically sealing ear mould it occludes the ear canal. This causes an elevation of the sound level of the user's own voice at the eardrum in the lower frequencies. For many hearing aid users their own voice then sounds hollow or boomy, and this is known as the Occlusion Effect (OE). The OE can be perceived so annoying to the user, that it becomes a major obstacle in the hearing aid use.
Blocking or occluding the ear canal with an ear mould has different effects on the sound from external sources and on the sound from the wearers own voice. Sound from external sources propagates as sound waves through the air to the ear. Occluding the ear canal attenuates the sound pressure generated at the eardrum (typically most at higher frequencies and less at lower frequencies).
Sound from the user's own voice propagates not only through the air from the mouth to the ear. For the lower frequencies the vibrations in the throat and the sound pressure in the vocal tract also propagate as vibrations in the bone and tissue to the wall of the ear canal. These vibrations in the wall do produce a sound pressure at the eardrum as well. However, in the open (not occluded) ear, the air can easily flow in and out of the ear canal, and the sound pressure resulting from the vibration is generally low and hardly significant compared to the sound propagating through the air.
In the occluded ear the air is trapped in the small volume of the ear canal, and so the vibration in the wall results in a much higher sound pressure, often significantly higher than the sound pressure would have been in an open ear at lower frequencies. At the same time the sound propagating through the air is attenuated (mainly at high frequencies) by the ear mould. These effects may cause the user's own voice to be perceived as sounding hollow and boomy.
The Occlusion Effect (OE) is generally a function of the frequency, but also of what sounds are spoken (articulated). Several other factors impact the OE as well.
The acoustic sealing of the ear mould has a strong effect. Introducing a leakage or vent in the ear mould generally decreases the OE. This is the most common way for reducing the annoyance, but it has also undesired consequences (jeopardizing stability or amplification of the hearing aid). A vent is often provided in the form of a tube or canal extending through the ear mould or hearing aid housing, facilitating transmission of acoustic waves from one side to the other so that the ear canal is not completely blocked. The vent allows bone conducted sound to escape from the inner portion of the ear canal. The energy loss and the risk of acoustic feedback increase with increasing vent diameter when the vent length is the same. However, prevention of the occlusion effect imposes the requirement of a large vent diameter. On this background it is often relevant to measure the occlusion effect when fitting a specific ear mould or hearing aid housing to a hearing aid user. Knowledge of the specific occlusion effect can be used for adjusting the vent diameter to an optimum dimension when considering occlusion, energy loss and feedback in relation to the individual hearing aid user.
The insertion depth of the ear mould also has an impact on the occlusion effect. It is mostly vibrations in the soft tissue forming the first part (from the entrance) of the canal that causes the OE. So a deeper insertion of the ear mould blocks more of the vibrating wall resulting in decreased OE.
Furthermore, the OE is impacted by individual anatomy which influences both the volume of the ear canal as well as the level of the vibration.
These factors make it difficult to predict and assess the OE just by inspection. A measurement of OE is usually required.
Whether a particular OE is perceived annoying or not does not only depend on the magnitude of the OE. Also the actual hearing loss and insertion gain of the hearing aid as well as personal tolerance may impact the perception and possible annoyance. Yet, it is important to assess the occlusion effect in the process of analyzing how a hearing aid user perceives his/hers own voice.
In Situ Occlusion Effect Measurement
The Occlusion Effect is a time variant transfer function. The OE of a speaker's own voice is a transfer function between the sound pressures generated at the eardrum by the voice when the ear canal is occluded ear and the sound pressures generated at the eardrum by the voice when the ear canal is open.
      O    ⁢                  ⁢    E    =            p              drum        ,        occluded                    p              drum        ,        open            
This implies a transfer function between two signals which do not exist simultaneously. Furthermore the transfer function does not only depend on properties of these two configurations, but also on the actual source (the voice signal, i.e. what is being articulated).
As it may be difficult to repeat a voice signal accurately enough for a proper serial measurement, the OE may be estimated from other transfer functions based on signals that do exist simultaneously.
The OE can be expanded into the following three factor product (each factor being a transfer function):
      O    ⁢                  ⁢    E    =                    p                  drum          ,          occluded                            p                  drum          ,          open                      =                            p                      drum            ,            occluded                                    p                      ext            ,            occluded                              ·                        p                      ext            ,            occluded                                    p                      ext            ,            open                              ·                        p                      ext            ,            open                                    p                      drum            ,            open                              pext,occluded and pext,open are the sound pressures at a point outside the ear canal or outside the ear with the canal occluded by the ear mould, or with the canal open, respectively. The position may e.g. be at the side of the head above the pinna, where a Behind-The-Ear (BTE) hearing aid is typically placed.
If the two latter factors (i.e. pext,occluded/pext,open) and (pext,open/pdrum,open)) are known and time invariant, a measurement of OE can be performed by measuring the first factor (transfer function) and then multiplying with the two other factors.
If pext,occluded and pext,open are captured (i.e. measured by transforming an acoustical signal into an electrical signal) by a microphone, e.g. the microphone of a BTE hearing aid, and pdrum,open is captured by a probe microphone, both factors can be determined and examined. For the lower frequency range in which the OE is of most importance both factors are close to 1, both factors show only little dependence of the speech signal and both factors show only little individual variation. So these two factors can be well approximated by constants. For the frequency range of interest this may also be generalized to apply to microphone positions of other types of hearing aids, e.g. In-The-Ear (ITE) or Completely-In-Canal (CIC) hearing aids.
So, the remaining task is to measure (pdrum,occluded/pext,occluded) for the actual individual in order to quantify the occlusion effect.
It is advantageous to be able to apply the hearing aid for the occlusion effect measurement. Such in situ occlusion measurement by application of the hearing aid gives a simple and fast measurement with minimum requirements for equipment to be applied in connection with the fitting of the hearing aid.
Depending on the purpose of the measurement different speech signals from the speaker may be used. Possible speech signals may be running speech as well as sustained articulation of specific vowels.
A convenient way of measuring this is by capturing pext,occluded by the hearing aid microphone and capturing pdrum,occluded by the hearing aid receiver.
2. The Prior Art
WO-A1-2008/017326 describes occlusion effect measurement by using the hearing aid, relying on the users own voice as a sound source. WO-A1-2008/017326 also discloses using the receiver (i.e. a loudspeaker) as the transducer measuring the sound pressure in the ear canal of the occluded ear. Thereby, the need for an extra microphone in the ear mould or hearing aid housing is avoided. The standard microphone is used for measuring the sound pressure outside the ear.
WO-A1-2008/017326 does, however, not disclose any information on how to use the receiver as the transducer. The receiver when used as transducer for measuring the sound pressure will give a very different response compared to a standard microphone used in a hearing aid. This is a problem since the two microphones needed for measuring the occlusion effect in situ should give the same response for the same sound pressure. Furthermore, the sensitivity of the receiver when used as microphone is considerably lower compared to a standard microphone.